1. Field of the Invention
The present invention relates to image forming apparatuses, particularly to copiers, facsimiles, printers and complex machines having these functions using electrophotographic processes.
2. Discussion of the Background
Recently, stable image quality which is free from uneven image density even when high-density or full-color images having a large image area are continuously produced is demanded.
Therefore, the image developer needs to separate and collect the developer from the developer bearer (herein after referred to as a developing sleeve) after producing images having a large image area consuming a large amount of the toner, feed the toner to the developer and uniformly disperse the toner therein to resume the original toner concentration, and quickly feed the developer to the developing sleeve.
However, in a conventional image developer as shown in FIG. 7, since a part separating the developer used for the development from the developing sleeve 2 and collecting the developer to a developer feeder 11 (hereinafter referred to as a screw) is close to a part feeding the new developer to the developing sleeve, the used developer is fed to the developing sleeve 2 again without a new toner, resulting in uneven image density.
In addition, since the collected developer and the developer being fed are mixed in a same section 13, the developer is difficult to have a uniform toner concentration between upstream and downstream sides of the screw 11 in FIG. 8. The developer has less toner concentration downstream and images having uneven image density in the longitudinal direction of the developing sleeve 2 are likely to be produced.
As shown in FIGS. 5 and 6, Japanese published unexamined patent application No. 5-333691 discloses a marketed functionally-separated image developer in which a screw and a feed route having been laterally located are vertically located to separate sections of feeding and collecting the developer, i.e., the developer is separated and collected by a lower screw 5 at a section 8 and the developer is fed by an upper screw 4 at a section 7.
However, at a communicating route D where the lower screw 5 transfers the developer to the screw 4, the developer needs to be deposited and the developer is fed from the section 8 to the section 7, i.e., the used developer is directly fed to the developing sleeve 2 again, resulting in uneven image density.
In addition, only the lower and upper screws do not fully stir the developer, resulting in uneven image density and deterioration of image density. Japanese published unexamined patent application No. 11-167260 discloses an image developer as shown in FIGS. 3 and 4, further including a section 9 besides the sections 7 and 8, having a stirrer 6 mixing and dispersing the collected developer and a toner fed from T to improve uniformity of the image density.
The two screws vertically located in FIGS. 5 and 6 save space more than the conventional two screws laterally located. However, as mentioned above, the developer deposited is fed to the developing sleeve again or is not well mixed with a newly-fed toner, i.e., the developer is not fully stirred, resulting in uneven image density and deterioration of image density.
In order to solve this problem, three screws are effectively located as shown in FIG. 3. The developer needs to deposit at a stirring detour 9 apart from the developing sleeve, the low-concentration developer just after collected is not fed to thereto again. Further, the collected developer and a newly-fed toner are sufficiently stirred at the stirring detour 9, which improves uneven image density.
On the other hand, the image developer has a toner concentration sensor detecting a concentration of the developer. This is typically a sensor detecting a magnetic permeability of the developer and detects a toner concentration of a specific amount of the developer close thereto. The sensor power and the toner concentration have a linear relationship each other, and the sensor detects excess and deficiency of the toner to drive or stop a feeder of the developer.
In order to detect whether a proper amount of the toner is fed, it is necessary to detect the developer fully dispersed and mixed and a specific distance is required between a position from which the toner is fed and a position the toner concentration is detected.
However, it is impossible to reduce the toner from or add the toner to the developer therebetween even when the toner is excessively or insufficiently fed from the position from which the toner is fed. Therefore, the developer has a part having a high toner concentration and a part having a low toner concentration while stirred and transferred, resulting in uneven image density after all.
In addition, polymerization toners frequently used lately are difficult to mix and disperse in image developers due to their shapes. This is because the polymerized toner having the shape of almost a sphere and a small particle diameter takes more time to mix with a developer in the image developer than conventional pulverization toners having a large particle diameter.
The toner concentration detector typically measures the concentration of the toner in a developer fully mixed. However, when the developer insufficiently mixed is detected, excessive toner feeding is repeated, resulting in toner scattering, background fouling and uneven image density.
Because of these reasons, a need exists for an image developer and an image forming apparatus capable of improving the dispersibility of a developer even including a toner having poor mixability and dispersibility, and precisely detecting the toner concentration to prevent uneven toner concentration of the developer and uneven image density.